Thermally controlled switch



2, 1958 w. K. BOUWMAN 2,847,538

'THERMALLY CONTROLLED SWITCH Filed Feb. 18, 1957 INVENTOR WlCHERKORNELIS BOUWMAN AGENT United THERMALLY CONTROLLED SWITCH ApplicationFebruary 18, 1957, Serial No. 640,921

Clains priority, application Netherlands' September 25, 1956 3 Claims.(Cl. 200-137) This invention relates to thermally controlled switchesintended for controlling the temperature of electrical apparatus suchas, for example, smoothing irons. These switches comprise two contactswhich are controlled by a thermo-responsive Operating element whichtends to open the contacts with an increase in temperature, and also bya current-dependent Operating element which urges the Contacts in thedirection of closure during the passage of current through the switch,the time constant of the therrno-responsive Operating element exceedingthat of the current-dependent element. It is pointed out that this doesnot preclude that the thermo-responsive element is also slightlycurrent-dependent if current were to pass through it and thecurrent-dependent element be slghtly temperature-dependent.

In conventional devices, the thermo-responsive operating element is abimetal strip but, fundamentally, any thermo-responsive element may beused, the time constant of which exceeds that of the current-dependentelement.

It is pointed out that, if this greater time constant is also caused byparts of the apparatus, the temperature of which is controlled, theseparts are associated with the switch.

The current-dependent operating element should be fast-acting. In knownswitches, a hot wire is employed.

The switches referred to operate as follows:

If the temperature of the apparatus to be controlled exceeds a givenlimit value, the opening of the Contacts is slowly initiated by theaction of the thermo-responsive Operating element having the higher timeconstant. However, the second element responds rapidly to any decreasein current and further opens the Contacts so as to reduce penetration.If the temperature drops below another limit, the element having thehigher time constant will slowly close the Contacts so that the contactpressure is intially low. The element having the lower time constantwill, however, instantly respond to the first passage of current andcause an increase in contact-pressure.

In known switches of this type, the thermo-responsive element with thehigher time constant was also connected in the electric circuit, hencethis element required to be isolated from the apparatus to becontrolled. On the other hand, this passage of current was useful and'desrable in order to make up for thermal lag on account of the isolatedarrangement.

The present invention is based on the realization that in this mannercomplete compensation is not feasible and, moreover, this manner ofcompensation makes it necessary to impose conditions on the elementhaving the higher time constant with regard to specific resistance,length and cross-section, also in accordance with the value of thecurrent to be interrupted. Hence, a compromise is ditficult to make. Forexample, a smoothing iron rated for 110 volts would require a bimetalstrip -tates Parent O different from that for use in a smoothing iron of220 volts.

In accordance with the invention, the Operating element with the highertime constant is insulated from the electrcal circuit and electricallyand mechanically connected to the apparatus, the temperature of which isto be controlled, since it consists of a tautened wire which is Securedat both ends to a base, the coefficients of expansion of the wire andthe base being different and the excursion of the centre of the wirebeing transmitted to the Contacts by, means of an insulator. This basemay, for example, be the sole of a smoothing iron. lt will, however, beappreciated that the invention is not to be taken as being liimted tothe precise constructional details referred to in the foregoing,particular description.

In these constructions, consequently, the current does not at allinfluence the element with the higher time constant, and the latter isable to follow only with a slight thermal lag the temperature of theapparatus to be controlled, since it is firmly Secured tothis apparatuswithout any insulation. The lag may practically completely be avoided ifthe wire is made from a material, for example that commercially sold aslnvarj the coefficient of expansion of which is substantially equal toZero, since in this case the therrno-responsive Operating elementresponds to the thermal expansion of a part of the apparatus itself tobe controlled, for example to expension of the sole of the smoothingiron. otherwise, such thermo-responsive elements are known per se.Furthermore, it is pointed out that in the present case the term "wire"is to be put broadly and also includes tape or rods.

In order that the invention may be readily carried into eect threeexamples will now be described in detail with reference to theaccompanying diagrammatic drawing, in which Fig. l is a side elevationView of the switch,

Fig. 2 is a detail View of another form, and

Fig. 3 shows a third construction in side elevation.

The reference numerals 1 designate two parts of an apparatus, forexample the sole of a smoothing iron, the temperature of which is to becontrolled within two predetermined limits. Stretched between theseparts 1 is an Invar strip 2 carrying an insulator 3 at the middle. Inthis case, the sole of the smoothing iron jointly with the "Invar stripconstitutes the thermo-responsive element having the higher timeconstant.

Secured to the insulator 3 are a hot wire 4 and a fiexible conductor 5leading to an insulated junction contact 6. The other end of the hotwire 4 is Secured to a spring 7 by which the wire 4 and the Invar' strip2 are tautened. The strip 7 furthermore carres a contact 8. A secondcontact 9 is secured to a spring 10, one end of which is connected to ajunction contact 11 and the other end of which is provided with anadjustment screw 12 co-acting with an eccentric control knob 13.

The operation of the hot wire constituting the thermoresponsive elementwith the smaller time constant, and the operation of the Contacts havealready been set out in the foregoing.

It will be evident that the thermo-responsive element constituted by theparts 1 and 2 will react without any lag to the temperature of theapparatus and is entirely independent of the value of the currentconsumed by the apparatus. This permits, for example, this thermostat tobe used in smoothing irons rated for widely varying voltages, whichconsequently consume widely varying Currents, without thethermo-responsive element itself needing to be modified.

It will be evident that the invention is not limited to the use ofthermo-responsive elements of the aforesaid type. The thermostatconstituted by the parts 1 and 2 may, for example, be replaced by abimetal strip which is directly connected to the apparatus to becontrolled, as shown in Fig. 2. In this case, a bimetal strip 20 isSecured by means ofscrews 21 to the sole 22 of a srnoothing iron. Theend of the strip carries the insulator 3 and the'remainder-of the switchmay be made up similarly to Fig. 1.

In the switch shown in Fig. 3, the thermo-responsive Operating elementis an Invar' rod 30 which is Secured to a component part of theapparatus to be controlled, for example .the aluminium-cast sole plate31 of a smoothing iron. In front of the end of the rod 30 is situated alug 32 which is .also secured to the sole, while the end 'of a bentlever 33 carrying an insulator 3 at its free end is .clamped between.the lug 32 and the rod 30. This insulator is engaged by a plate spring34 acting in the direction of the arrow 35. The spring 34 furthermorecar'ies a contact36 and amagnet armature 37. A second spring 38 carriesa contact 39. The positon of this spring is adjustable by means of aneccentric 40. The armature 37 `can be attrac'ted by an electro-magnet41.

The operationof this switch is as follows: If the sole 31 of thesmoothing iron cools down, the space between the end of the rod 30 andthe lug 32 is reduced, and the insulator 3 lifts the spring 34, -so thatthe contacts 36 and 39 approach each other slowly. Upon contact beingestablished, a circuit is completed from 'a terminal 42 via spring 34,contacts 36 and 39, spring 38, conductor 43,rnagnet coil 41 to aterminal 44. The armature 37 and the coil 41 constitute thecurrent-dependent operating element by which the contacts are rmlyclosed so that the contact 39 Will betslightly tilted.` When ;reachingthe upper temperature `limit the operation reversed: as :soon :as thecontacts are slowly opened'the magnet be- 4 comes 'less energized sothat the contacts -are instantly completely separated.

What is claimed is:

l. A thermally controlled electric switch for electrical apparatushaving a base member and an insulator com prising a thermo-responsiveOperating element which opens said contacts upon the reaching of apredetermined temperature level, a current-dependent Operating elementwhich urges the contacts in the direction of closure during the passageof current through said switch, the time constant of thethermo-responsive Operating element exceeding the time constant of saidcurrent-dependent operating element, said thermo-responsive Operatingelement being insulated from said electric circuit and beingelectrically and mechanically connected to said apparatus, saidthe'rmo-responsive Operating element including a tautened wire Securedat each end to said base member, the coefficients of expansion of saidbase member and said wire being different and the movement of a portionof said wire being operatively transferred to said contacts by means ofsaid insulator.

2. A thermally controlled electric switch as claimed in claim l wherensaid base is a sole of a smoothing iron.

3. A thermally controlled electric switch as claimed in claim l wherensaid current-dependent Operating element is an electro-magnet.

References Cted in the file of this patent UNITED STATES PATENTS2,584,9,24 Reingruber et al. Feb. 5, 19.52 i 2,7 l6172 Franklin Aug. 23,1955 2,7l6,173 Russell Aug. 23, 1955 UNITED STATES PATENT OFFICECERTIFICATE OF CFDRRECTION Patent No 2,84'7538 August la 1958 WieherKornelis Bouwman It is hereby Certified that error appears in the abovenumbered patent requiring correction and that the said Letters Patentshould read as COT reeted below In the heading to the prntedspeoifieation line 9, for the priority date, "September 25, 1956" readFebruary 25, l956 Signed and sealed this 9th day of December 195&

Attest:

H AXL KARL INE ROBERT C WATSON Attesting officer Corrmissioner ofPatents UNITED STATES PATENT OFFICE CERTIFICATE OF CRRECTION Patent No.,2,847,538 August 12 l958 Wieher Kornelis Bouwmar It is hereby Certifiedthat error appears in the above numbered patent requrng correction andthat the said Letters Patent should read as cor rected belowo In theheading to the printed specifcaton, line 9, for the priority date,"September 25, 1956" read February 25, 1956 Signed and sealed this 9thday of December 195&

Attest:

KARL Ho AXLINE ROBERT C. WATSON Attesting officer Commissioner ofPatents

